Securing devices have become increasingly important with the continuing increase in theft crimes. As a result, locking devices of various kinds have been developed to meet various needs in protecting against such crime. Despite these developments, the basic structure of the locking mechanism in devices for securing motor bikes or other vehicles has remained relatively unchanged. For instance, a prior art locking mechanism, as shown in FIGS. 4A, 4B, 5A, and 5B, comprises an elongate lock housing A having an axial bore adapted to receive a lock core B and a side opening for insert of a lock rod assembly C therethrough. The lock core B has an integral engaging plate B1 extending from an end portion thereof. The lock rod assembly C has a recessed portion at a lower part thereof and a dome C1 centered on one face of the recessed portion. The dome C1 is blocked by the engaging plate B1 from moving upward when the engaging plate B1 is in its locked position. A clearance exists between the engaging plate B1 and the dome C1 when plate B1 is in that locked position, resulting in loose engagement of the plate B1 with the dome Cl.